plans 87, 88: tighten failure-mode and CommonMark wording

Address two Copilot review points:

- plan 87: described the non-GFM render of `> [!NOTE]` as a
  "silent failure", but the literal marker text is visibly
  rendered — that is a visible failure. Reword to match.
- plan 88: the phrase "None are part of CommonMark, GFM, or
  goldmark" was ambiguous — `[TOC]` IS valid CommonMark syntax
  as a shortcut reference link. Reword to make clear that none
  of the four tokens are standardized as TOC directives, while
  `[TOC]` remains a valid (but differently-resolved) link
  shorthand.

Author: claude

plan/87_markdown-flavor-github-alerts.md

@@ -47,8 +47,10 @@ renders as literal text inside a blockquote:
 
 becomes a blockquote whose first line is the
 literal string `[!NOTE]`. The author intended a
-styled callout; the reader sees unstyled text.
-This is a silent failure.
+styled callout; the reader sees unstyled text
+with the marker token visible inside the
+blockquote. The failure is visible, not silent,
+but the author's intent is still lost.
 
 ### Why not a generic container rule
 

plan/88_toc-directive-migration.md

@@ -35,14 +35,22 @@ Four TOC directive variants appear in the wild:
   VitePress
 - `${toc}` — some VitePress configurations
 
-None are part of CommonMark, GFM, or goldmark.
-On those renderers the directive does not
-expand into a TOC; it renders as literal text.
-The exact output depends on the pattern:
-
-- `[TOC]` without a matching link reference
-  definition renders as the literal string
-  `[TOC]` (goldmark emits a "no matching link
+None are standardized TOC directives in
+CommonMark, GFM, or goldmark. `[TOC]` is still
+valid CommonMark syntax — a shortcut reference
+link — so it only renders as literal text when
+no matching `[TOC]: <url>` definition is
+present; with a definition, it renders as a
+link. The other three tokens are not valid
+link-reference shorthand and always render as
+literal text. On those renderers no token in
+this set expands into a generated TOC.
+
+Concrete render outputs when no link reference
+is present:
+
+- `[TOC]` renders as the literal string `[TOC]`
+  (goldmark emits a "no matching link
   reference" fallback, which is verbatim text)
 - `[[_TOC_]]` renders as `[[_TOC_]]` inside a
   paragraph